There has been proposed a process for producing information recording discs of an air sandwich structure made of plastics to perform storing and reading of information by converging rays of light such as laser beam and the like on recording medium containing layers, which process comprises subjecting two disc substrates through outer and inner peripheral spacers lying therebetween to ultrasonic welding (e.g. Japanese Patent Laid-Open Publn. No. 103537/1985).
The proposed process is intended to laminate two disc substrates to each other, wherein outer and inner peripheral spacers both having projections on both surfaces thereof are laid between said two disc substrates, ultrasonic wave vibration emanated from an ultrasonic welding machine is applied through said two disc substrates to said outer and inner peripheral spacers, and the projections of said outer and inner peripheral spacers are softened, thereby fusion bonding said two spacers to said disc substrates at once. This process has such advantages that the process is simple and the rate of production obtained is high in this joining process.
In the prior art process as referred to above, however, there were involved such problems that shear in positioning of disc substrates and outer and inner peripheral spacers tends to occur when ultrasonic wave vibration is transmitted thereto, particularly because of its marked deformation of elastic after-effect, the outer peripheral spacer is liable to squeeze out from or enter inside the peripheral portion of disc substrates, and the positioning for obtaining a laminated disc having fusion bonded the spacers to the positions becomes difficult and, at the same time, even when the spacers have been fusion bonded to the predetermined positions, warp occurs on the disc surface and no information recording disc having an angle of warp in conformity to the standard specification (within 5 mrad according to ISO) is obtained.